Method for the production of casting moulds



METHOD FOR THE PRGDUCTION OF CASTING MOULDS Clifiord Shaw,Hildenborough, Tonbridge, England, assignor, by mesne assignments, toShaw Process Development Corp, Port Washington, N. Y.

No Drawing. Application March 8, 1954,

a Serial No. 414,884

Claims priority, application Great Britain April 1, 1953 15 Claims. (Cl.22-193) This invention generally relates to metal castings andespecially to the method of producing casting moulds for effectingprecise castings, and is a continuation-inpart of co-pending applicationSerial Number 347,829, filed April 9, 1953 by Noel Shaw, entitled, MetalCasting, Moulds Therefore and Their Production.

That application covers a method of producing casting moulds wherein themould material is prepared in situ from a mixture composed of finelyground refractory material and a binder, of a low alkyl silicate type,such as ethyl or methyl silicate and an aqueous acid gelling agent, andin which mixture hydrolysis of the binder is effected, whereby alcoholis formed which escapes from all surfaces of the mould. That alcohol isignited the moment the surfaces of the mould have set by gelling.

The ignition and burning of the alcohol at all outer mould surfacesrapidly hardens these surfaces and effect their rapid solidification andsetting, thereby preventing changes in the external mould dimensions. Asthese outer mould surfaces set and harden,-the interior mass or body ofthe mould material contracts independently of the set outer mouldsurfaces so that their dimensions remain unchanged. That contraction ofthe interior mould body manifests itself by a uniform formationthroughout the mould body of numerous microfissures, thereby renderingthat body uniformly porous. That porosity of the mould is highlydesirable when the mould is used for metal casting in that that porosityobviates the provision of heretofore required vents for the removal ofair and gases. In addition to the fact that by the aforesaid method theexact external dimensions of the mould are preserved, these moulds proveremarkably stable to heat shock or abrupt fluctuations of temperature.

The above outlined method is confined to the use of an alkyl silicatebinder, which changes through hydrolysis into a volatile, combustiblealcohol. That liquid is then evaporated from the mould body, and sinceit is combustible, it is ignited and thus conveniently starts a rapid,simultaneous and uniform heating of all the external mould surfaces,thereby causing a fixating or freezing of the external mould dimensions.

It has now been found that the principle of fixating or freezing of theexternal mould dimensions by a rapid and moreover uniform heating of thetotal surface of the mould can be applied to moulding mixturescontaining binders which do not yield combustible volatiles uponsetting. In the preparation of such casting moulds liquid, semi-liquidor paste-like substances aroused which do not evaporate readily. Theseare subjected, when the mould has set, to a heat shock, that is anintensive and rapid heating applied simultaneously and uniformly to allmould surfaces, thereby forcing any evaporable substances contained inthe mould mass to rapidly escape. That heat shock is effected the momentthe mould has gelled, but has not yet appreciably shrunk due to slowevaporation of liquids in the mould mix. This aforesaid method ofintense and rapid generation of heat applied to the mould J nited StatesPatent Patented Nov. 5, 1957 does not depend upon the combustion of anycombustible components, since by definition there are none.

According to the present invention, therefore, in the preparation ofcasting moulds for metals or other mould bodies, a pattern is envelopedby a liquid, semi-liquid or plastic moulding mixture consisting offinely ground refractory material and a liquid binder. Such mouldingsubstance is poured into a moulding box, is allowed to gel i. e. to set,so that it can be handled without causing damage to it, and then thepattern is removed from the mould. According to this invention, themould or the aforesaid mould body is subjected to a heat shock by rapidand uniform application of intense heat to all the surfaces of themould, immediately after it has set, but before an appreciable shrinkagecaused by evaporation has occurred. Evidently the temperature applied tothe surfaces of a mould prepared from the aforesaid moulding mixturemust be considerably higher than would be required for the liberation ofcombustible volatile components. Upon the applied heat shock, animmediate and complete hardening of the external mould dimensions takesplace as well as a uniform formation of micro cracks in the internalstructure of the mould mass, thus rendering the latter uniformly porous.

The invention can be employed with advantage in the preparation ofcasting moulds in using a moulding substance which contains finelyground refractory materials and a binder, the latter being preferablyformed by precipitation of an inorganic silicate, such as potassium oranalogous silicates, whereby the conversion of the formed sol into a gelis effected. In the moulding compound there can'also be employed otherliquids and setting binders, for example, binders obtained by means ofmagnesia and phosphoric acid, both well known in the art. "The inventionmakes it possible to prepare accurate mould without employing the slow,time consuming drying method, considered to be necessary in heretoforeused mould making. Much time and a considerable storage space during thedrying period of the moulds can be saved hereby. Furthermore, mouldsprepared according to the present invention possess an increasedresistance against abrupt temperature changes.

In carrying out the invention in greater detail, refractory materials,as for example alumina silicate, magnesia, clay, fused alumina, powderedsilica, zirconium dioxide and zirconium silicate, etc. are worked-upinto a mouldable slurry by means of an aqueous base liquid binding agentand used for the preparation of a mould around .a pattern. After thesetting, and if so desired, after separation of the mould into separateparts, this mould is placed on a grate or is in other ways arranged orsupported so that it is possible to apply simultaneously and rapidlyintense heat from all sides, and so that volatiles will be' forced toescape rapidly and as much as possible completely. For example the mouldon the grate is put into a muffle furnace which is pre-heated to atemperature of approximately 700 to 800 degrees centrigrade', which isconsiderably higher than would be necessary for the simple drying of themould, or the mould is subjected to directed and controlled series ofopen flames.

By way of example thefollowing materials are mentioned as binding agentswhich may be employed in carrying the present invention into effect:inorganic binding mediums, such as inorganic silicates, especiallysodium silicate and potassium silicate (the latter particularly in theform of aqueous sols) and phosphates.

The period of time which'may be allowed to elapse between the settingand the rapid heating of the mould depends on the volatility of theevaporating liquid medi, um, whereby the principle is to be borne inmind that any shrinkagew by evaporation is to be avoided before theexternal surfaces are fixed or frozen by the rapid heating. Since in allthese cases the volatile component is water, this interval of time isnot as critical as in the case of the aforementioned method where thevolatile component employed is an organic liquid, such as alcohol, whichby definition evaporates more quickly than water.

The present invention can also be used with advantage in connection witha method in which before the baking of the mould the surfaces of thelatter are insufflated with a fine metal powder passing for instance aBOO-mesh sieve, in order to assist the formation of nuclei in thecasting metal on the surface of the mould. Such insuffiation is carriedout While the mould surface is still moist, or after it has been coveredwith a suitable binding agent.

The fine metal powder above mentioned is a matter of choice and dependsupon the metal or metal alloy to be cast. Thus for certain castingsaluminum powder has been found useful, whereas for other castingsnickel, copper, magnesium or other metal powders may be employed.

The reactions taking place in carrying out the present invention are notclarified in all details. Nevertheless it can be stated without limitingthe present invention in any way, that through the uniform and rapidapplication of intense seat to all surfaces of the mould, these surfacesare brought to a rapid, complexe fixation and hardening, so that thedesired external dimensions of the mould remain unchanged.

During the rapid hardening of the exterior mould surfaces, there occurin the interior mass or body of the mould fine fissures and crevicesthroughout, which are so minute that in the process of casting the metalcannot infiltrate, while on the other hand these microfissuresfacilitate an unrestricted escape of gases and vapours during thecasting process.

in order to achieve such microfissure formation in the mould body, theremoval of volatiles after the mould has been formed and when ithas setshould take place as rapidly as is physically possible. As a result, theground mould mass cracks uniformly throughout the body and the cracks ormicrofissures are very fine and are formed in such a manner that theminute fragments of the mould body separated by these microfissures aresubstantially of the same order or size. The individual fragments soproduced are of a complicated form and are not rigidly joined togetherbut are interlocked and dovetail; nevertheless the mould as a wholebehaves like a solid body,

while on the other hand the expansion and contraction of the individualfragments forming the internal mould body structure is renderedpossible, without disturbing the external dimensions of the castingmould as a whole.

As stated, the microcracking of the internal mould structure is uniformand even and must be so, since if it is not uniform and even distortionof the mould body will take place, unless it is of the simplest form.

According to the present invention it is essential that the particles ofcomminuted refractory be separated by a colloidal ground mass or gelledstructure. This clearly distinguishes in principle over previoustechniques in which an essential objective is the consolidation andcompacting of the refractory particles by vibration or otherwiseaimingto achieve as nearly as-possible complete contact between them. This istrue both for sand casting and precision lost wax casting, as Well asbeing normal practice in the refractories industry in general and indeedin all the art in which it is sought to bind together aggregate. It isan essential in achieving the presently desired opposite aim thatsetting or gellation of the mass as such should take place with aminimum of disturbance, since such disturbance tends to cause compactingof the refractory particles, which is very undesirable as it leads todistortion of the mould body.

According to the present invention it is necessary to prepare a mobileslurry containing a substantial amount of liquid, obtained by mixingcomminuted refractory material with a wet binder. The thus preparedmobile slurry is then poured over patterns placed in suitable containersas is well known in the art. As soon as the slurry has set, andaccording to the conditions described hereabove, the non-combustiblevolatile is rapidly removed by the application of a proper thermalshock, as, for instance, by placing the gelled mould into a pre-heatedfurnace.

The binders may be prepared, as is Well known in the art (see forexample Collins U. S. Patent No. 2,380,945), from inorganic silicates,such as for example: sodium silicate and an acid such as nitric acid, orweak alkaline cclioidal silica solution and a gelling accelerator; or aweak acid colloidal silica solution and a gelling accelerator; or, as iswell known in the art, from phosphates such as, for example, ammoniumphosphate and magnesium oxide in a water medium. In each case, once theslurry has set, the resulting volatile which is Water must be rapidlyremoved from the mould as described.

The rapid, high temperature heating of the mould can be possiblyefiected by the introduction of the mould into a chamber heatedelectrically, as by induction or resistance.

By the present method the preparation of a casting mould with anextraordinarily smooth casting surface is made possible, without the useof multi-layered moulds. However, it is also possible to prepare themould in layers and to use for the casting surface especially finelygrained refractory materials, a fact which is generally known. It isalso possible to strengthen the binding effect between the castingsurface and the mould body material by an intermediate connecting layeror by a suitable choice of various binding media. For example it ispossible to use as casting surface a moulding substance containingalkaline silica sols, While using for the remaining body I of the moulda weak silicic acid sol as binding agent.

The compound formed by the neutralization of the two agents producesstrong adherence of the casting surface to the main body of the mould;

Becoming more specific as to the composition of the slurry and thetemperatures required to produce the intended results of fixating orfreezing the outer dimensions of the mould and effecting a uniform andevenly distributed micro-cracking of the internal body structure of themould, the following typical example will apply:

Refractory material Good results were obtained by the use of comminutedsillimanite (kyanite) prepared in the following typical grading:

Sieve Size Percentage ranging from to +16 ranging from 1G to +22 rangingfrom -22 to +30. ranging from to +44. ranging from -44 to +60. rangingfrom to +100. ranging from to +150. ranging from to +200. ranging from2U0 -s Liquid medium According to the present invention an aqueousgelling medium is used. For example, a solution is prepared containingmetallic silicate, such as potassium silicate, which is mixed with waterto a concentration of approximately 60 degrees Baum.

The above given refractory material is mixed with the aqueous gellingmedium in approximately the following proportion:

Gelling medium 110 Refractory material, including the controlling agentgrms 132 These two ingredients are mixed to form a slurry which remainsmobile for approximately 45 seconds, during which time it is pouredeither over a pattern or into a container into which the pattern isplaced.

After the above stated period the mixture suddenly gells, and while inthat stage the pattern is removed without damage or distortion of theimpression made by the pattern.

Since the mould composition is produced with an aqueous base,evaporation of the water contained therein is relatively slow andtherefore there occurs no appreciable shrinkage between the time of theremoval of the pattern and the firing of the mould.

The firing of the mould The stripped'mould is now placed in a pre-heatedmuffle furnace kept at temperatures either between 500 to 600 degreesCentigrade for moulds of small or medium sizes for producing castings upto say 15 pounds in Weight, or between 700 and 800 degrees centigradefor larger sized moulds, at which temperatures the water content of themould material is vigorously, and therefore rapidly removed. As a resultof the rapid evaporation of water from the mould material the outersurfaces of the mould become fixated or frozen at the dimensionsimparted to the mould by the pattern, whereas the interior bodystructure of the mould becomes uniformly crazed or cracked. Thatinterior micro cracking is due to the fact that the comminuted particlesof the rafractory material are originally physically separated from oneanother by the aqueous gell, and these particles remain separated afterthe mould is fired, and the water is driven off from the gell. However,these refractory particles become interlocked and are dove-tailed withone another, but due to their physical separation, the interiorstructure of the mould formed by these interlocked particles can readilycontract or expand without affecting the exterior surface dimensions ofthe mould.

A mould produced according to the above outlined example is particularlyintended for casting metals or alloys of a relatively low melting point,that is a melting point lower than those of steel or steel alloys. Thereason for this limitation is that the mould thus produced is notsufficiently refractory for metals or alloys of high melting points.

Alternative application When it is desired to use the above mouldcomposition, which is relatively inexpensive, for high melting pointmetals and alloys, there has been found a useful employment of thatcomposition as a support or outer envelope for a highly refractory innerlayer, facing or lining which will withstand high melting pointtemperatures. Such lining or facing can be produced in accord- ;ancewith the first-mentioned co-pending application Serial Number 347,829.That composition would contain, for example, an ethyl silicate baseslurry which is prepared by mixing comminuted sillimanite of thepercentual grading given in the above example, with a liquid medium inthe form of an ethyl silicate, having a silica content ranging from 40%to 45%, and which ethyl silicate is hydrolyzed in association withdenatured alcohol and distilled water, the above effected liquid mediumconstituting a colloidal solution of silica in alcohol.

Comminuted sillimanite grn1s to 32 0 Liquid medium ccs 100 Gelling agentccs 5 After having thoroughly mixed these ingredients, the resultingslurry is poured over the face of the pattern or model and forms, whengelled, a thin skin or coating of say one-thirty-second of an inchthickness. The gelling of that slurry will take approximately one and ahalf minutes, and while the gelling takes place the coating may bedusted with a coarse sillimanite powder (10+30) to furnish a keyed backor anchorage for the outer aqueous base slurry given in the firstexample, which is now applied as an envelope or backing over the ethylsilicate base coating or facing. When that backing has gelled the mouldis stripped in the usual manner. As a result of the use of the twoslurry compositions there is produced a mould structure having a sturdybut inexpensive outer layer which supports and reinforces a highlyrefractory facing for all critical casting areas of the mould structure.

From the foregoing description of the present method it becomes quiteevident that for producing the desired mobile slurry for low meltingpoint castings it is required to mix comminuted refractory material witha wet hinder, or with a wet binder and wet gelling agent, from both ofwhich latter ingredients liquid matter is to be rapidly removed,however, neither of them are to contain combustible volatiles.

That rapid removal of liquid matter from the gelled mould effected by asevere heat shock applied uniformly and simultaneously to all mouldsurfaces is dependent on to produce a mould characterized by thefeatures of retaining exactly its originally imparted to externaldimensions and having a uniformly porous interior structure. j

Obviously both the choice of refractory material as well as the choiceof the binder and the gelling agent will largely depend upon the metalsto be cast in the mould so that the most suitable mould ingredients forthe best production of specific castings are employed.

While a series of examples of binders are given herein, it is evidentthat the present invention is not to be restricted to just the fewingredients stated, and that variations and changes in the compositionof the slurry may be effected, depending upon the type of material to becast, such changes or modifications being deemed to reside within thebroad scope of this invention, as defined in the annexed claims.

What is claimed as new is:

1. Method of producing refractory moulds which com-- prises preparing anaqueous slurry comprising a refractory filler and an inorganicnon-flammable binder and a gelling accelerator, pouring said slurry overa pattern, allowing the slurry to gel, immediately separating the gelledmass of the slurry from the pattern, and immediately heating the gelledmass to rapidly drive off water in vapor form, thereby to fix theexternal mould dimensions and obtaining a mould having an interiorstructure of micro-cracks which will not allow cast metal to fill thecrack formation, but which will facilitate the ready escape of gasesduring the casting of metal.

2. Method of producing refractory moulds which comprises preparing anaqueous slurry comprising a refractory filler and an inorganicnon-fiammable binder and a gelling accelerator, pouring said slurry overa pattern, allowing the slurry to gel with a minimum of disturbance,immediately separating the gelled mass of the slurry from the pattern,and immediately heating the gelled mass to rapidly drive off water invapor form,

thereby to fix the external mould dimensions and btaining a mould havingan interior structure of microcracks which will not allow cast metal tofill the crack formation, but which will facilitate the ready escape ofgases during the casting of metal.

3. Method in accordance with claim 2 wherein the binder is an inorganicsilicate.

4. Method in accordance with claim 2 wherein the binder is sodiumsilicate.

5. Method in accordance with claim 2 wherein the binder is potassiumsilicate.

6. Method of producing refractory moulds which compn'ses preparinganaqueous slurry comprising a refractory filler and an inorganicnon-flammable binder and a gelling accelerator, pouring said slurry overa pattern, allowing the shury to gel, immediately separating the gelledmass of the slurry from the pattern, placing the separated gelled massupon a supporting surface to allow application of. heat to all sides ofthe said gelled mass, and immediately heating the gelled mass to rapidlydrive off water in vapor form, thereby to fix the external moulddimensions and obtaining a mould having an interior structure ofmicro-cracks which will not allow cast metal to fill the crackformation, but which will facilitate the ready escape of gases duringthe casting of metal.

7. Method of producing refractory moulds which comprises preparaing anaqueous slurry comprising a refractory filler and an inorganicnon-flammable binder and a gelling accelerator, pouring said slurry overa pattern, allowing the slurry to gel with a minimum of disturbance,immediately separating the gelled mass upon a supporting surface toallow application of heat to all sides of the said gelled mass, andimmediately heating the gelled mass to rapidly drive off water in vaporform, thereby to fix the external mould dimensions and obtaining a mouldhaving an interior structure of micro-cracks which Will not allow castmetal to fill the crack formation, but which will facilitate the readyescape of gases during the casting of metal.

8. Method in accordance with claim 7 wherein the binder is an inorganicsilicate.

'9. Method in accordance with claim 9 wherein the binder is sodiumsilicate.

10. Method in accordance with claim 7 wherein the binder is potassiumsilicate.

11. Method in accordance with claim 7 wherein the binder is asolprepared from sodium silicate.

12. Method in accordance with claim 7 wherein the binder is a solprepared from potassium silicate.

13. A mould as produced in accordance with the method of claim 2.

14. A mould as produced in accordance with the method of claim 12.

15. Method of producing refractory moulds which comprises preparing anaqueous slurry comprising a refractory filler and an inorganicnon-flammable binder and a gelling accelerator, pouring said slurry overa pattern, allowing the slurry to gel with a minimum of disturbance,immediately separating the gelled mass of the slurry from the pattern,and immediately heating the surface of said gelled mass to rapidly driveoff water in vapor form, thereby to fix the external mould dimensionsand obtaining a mould having an interior structure of micro-cracks whichwill not allow cast metal to fill the crack formation, but which willfacilitate the ready escape of gases during the casting of metal.

References Cited in the file of this patent UNITED STATES PATENTS1,981,403 Weitzenkorn Nov. 20, 1934 2,380,945 Collins Aug. 7, 19452,388,299 Thielemann Nov. 6, 1945 2',441,695 Feagin et a1 May 18, 19492,496,170 Mann Jan. 31, 1950 2,518,040 Mann Aug. 8, 1950 2,652,609 SudiaSept. 22, 1953 OTHER REFERENCES Iron Age, November 9, 1944, pages 56-58relied on.

1. METHOD OF PRODUCING REFRACTORY MOULDS WHICH COMPRISES PREPARING ANAQUEOUS SLURRY COMPRISING A REFRACTORY FILLER AND AN INORGANICNON-FLAMMABLE BINDER AND A GELLING ACCELERATOR, POURING SAID SLURRY OVERA PATTERN, ALLOWING THE SLURRY TO GEL, IMMEDIATELY SEPARATING THE GELLEDMASS OF THE SLURRY FROM THE PATTERN, AND IMMEDIATELY HEATING THE GELLEDMASS TO RAPIDLY DRIVE OFF WATER IN VAPOR FORM, THEREBY TO FIX THEEXTERNAL MOULD DIMENSIONS AND OBTAINING A MOULD HAVING AN INTERIORSTRUCTURE OF MICRO-CRACKS WHICH WILL NOT ALLOW CAST METAL TO FILL THECRACK FORMATION, BUT WHICH WILL FACILITATE THE READY ESCAPE OF GASESDURING THE CASTING OF METAL.